A conductive transparent electrode is an integral component of a photovoltaic cell. Indium tin oxide (ITO) is currently the most commonly used transparent electrode material. Although ITO offers excellent optical and electrical properties, the fabrication of an ITO electrode involves costly vacuum deposition techniques. ITO (and other metal oxides) also suffer from being brittle, and thus are incompatible with flexible substrates. Further, with the increasing costs of mined metals, ITO is becoming a less economically viable solution for large scale photovoltaic cell production.
Graphene is considered a leading candidate to replace ITO as the transparent electrode material in photovoltaic devices since it can be solution processed, which may significantly drive down the cost of photovoltaic device fabrication and allow for compatibility with virtually any substrates. As-prepared graphene sheets typically have a sheet resistance of from about 250 ohms per square (ohm/sq) to about 4,000 ohm/sq, depending on the fabrication process. To be useful as a transparent electrode material in photovoltaic devices, the sheet resistance of the as-prepared transparent graphene films needs to be reduced.
Two approaches can be pursued to reduce the sheet resistance of graphene: stacking of several graphene films on top of each other and/or chemical doping. Stacking of graphene films essentially adds additional channels for charge transport. However, this approach simultaneously reduces the transparency of the system. See, for example, Li et al., “Transfer of Large-Area Graphene Films for High-Performance Transparent Conductive Electrodes,” Nano Letters, vol. 9, no. 12, pgs. 4359-4363 (2009). In addition, since the electronic properties are essentially preserved in the stacked graphene films (SGF), alternative solutions such as doping must also be considered. See, for example, Jung et al., “Charge Transfer Chemical Doping of Few Layer Graphenes: Charge Distribution and Band Gap Formation,” Nano Letters, vol. 9, no. 12, pgs. 4133-4137 (2009) (hereinafter “Jung”).
Graphene is classified as a semi-metal or zero-gap semiconductor where the density of states vanishes at the Dirac point. Undoped graphene has a low carrier density, and thus high sheet resistance, due to its vanishing density of states at the Dirac point. Due to unintentional dopants the Fermi level most certainly will not reside at the Dirac point of chemical vapor deposition (CVD)-grown graphene films exposed to air, yet chemical doping should still inject sufficient carriers to reduce the resistance of the film. This can be accomplished by injecting charges that result in a shift in the graphene Fermi level without interrupting the conjugated network. See, for example, Jung. Doping of SGF shifts the Fermi level further away from the Dirac point leading to a large increase in the conductivity. See, for example, Voggu et al., “Effects of Charge Transfer Interaction of Graphene with Electron Donor and Acceptor Molecules Examined Using Raman Spectroscopy and Cognate Techniques,” J. Phys. Condens. Matter 20, pg. 472204 (2008), Lu et al., “Tuning the Electronic Structure of Graphene by an Organic Molecule,” J. Phys. Chem. B, 113, 2-5 (2009) and Eberlein et al., “Doping of Graphene: Density Functional Calculations of Charge Transfer Between GaAs and Carbon Nanostructures,” Phys. Rev. B, 78, 045403-045408 (2008). Stacking of graphene sheets leads to a reduction in transparency of the graphene films, which is detrimental for transparent electrodes. Current graphene doping techniques provide dopants that are not stable in time.
Therefore, improved techniques for reducing the sheet resistance of transparent graphene films would be desirable.